In building architecture, a widely used curtain wall construction is the so-called "butt glazing system" wherein vertical mullions do not extend through the curtain wall but rather are adjoined by the wall. The curtain wall panels, typically panes of glass, are adhesively bonded to the building structure in a process known as "structural silicone glazing." This construction permits large expanses of glass unbroken by metal framing elements, which can present an aesthetically desirable appearance.
One of the biggest problems associated with this type of construction is the glazing and weatherproofing of such a structure. Conventional glazing and weather stripping techniques require that the final bead of silicone weatherproofing between adjacent panes be applied from outside the building, which can necessitate the erection of exterior scaffolding and substantially increase labor costs. Additionally, the quality of the weatherproofing joint is largely dependent upon the craftsmanship of the worker applying the sealant. Accordingly, it is highly desirable to provide a system for structural silicone glazing which permits the glazing and weatherproofing to be accomplished entirely from the interior side of the curtain wall, and which provides an attractive and efficient weatherproofing seal without the requirement for highly trained labor.
Previous efforts have been made to provide such a system, one such effort being disclosed in U.S. Pat. No. 4,562,680. The system of that patent requires a specially configured frame member with a chevron-shaped front wall forming an angle of at least 135.degree.. A semicircular channel open along its forward portion is formed at the apex of the angled front wall. A special elongated mullion insert has a gasket formed along a front edge. The rear edge of the mullion insert has a T-shaped connector portion formed thereon. The T-shaped connector portion is wider than the opening in the front of the semicircular channel, such that the connector can be inserted into the channel only by introducing it at an angle and rotating the mullion insert into place. When so installed, the mullion insert is held within the semicircular channel by the width of the T-shaped connector portion being wider than the opening in the front of the channel. To install a curtain wall according to this system, a glazing panel is positioned against the end mullion and the adjacent interior mullion from inside the building. A mullion insert is then installed in the semicircular channel of the chevron-shaped front mullion wall and pivoted so that the gasket portion abuts the edge of the first panel. A second panel is then positioned against the opposite side of the mullion insert and against the next adjacent mullion. The glazing procedure is then repeated progressively.
While the system of the aforementioned U.S. Pat. No. 4,562,680 provides certain advantages over the prior art, it suffers certain drawbacks. First, because the mullion insert must be rotated into position, the glazing panels must be installed progressively, that is, the panels must be set in a particular sequence. This requirement of setting panels progressively not only makes original installation less convenient but also presents difficulties in reglazing, should it later become necessary to replace a panel. Also, because of the requirement that the mullion insert be rotated into place, this system will work only with mullions having a front face of a particular configuration, thereby rendering it impractical for adaptation to other mullion configurations. The mullion insert can be difficult to pivotably install in the semicircular channel. Finally, the resulting construction provides only a single gasket between the panels such that a failure of the single gasket to seal may permit air and water to enter the curtain wall.
A different approach is disclosed in U.S. Pat. No. 4,650,702, wherein each pane of glass of the curtain wall system has a prebonded structural interface adhered along its edges. The structural interface is clipped onto the face of the mullion during installation to fasten the pane to the mullion. The system further comprises a non-structural weather seal between adjacent panels which is installed from the interior side of the curtain wall and which is captured by the structural interface when the panel is clipped onto the mullion.
The aforementioned U.S. Pat. No. 4,650,702 affords certain improvements over the prior art but presents certain limitations. The mechanical interlocking between the structural interface and the face of the mullion requires a mullion configuration which is rather complex. Accordingly, this weather sealing system is not easily adapted to mullions of other configurations. Also, the installation of this system is very labor intensive and requires workers of skilled craftsmanship. This system also requires the panels to be set progressively, and the necessity that the panels be installed in a particular sequence can render it difficult to reglaze a panel from the interior of the curtain wall.
Thus, there is a need to provide a system for butt-glazing curtain wall panels wherein glazing and weatherproofing can be accomplished from the interior side of the curtain wall.
There is a further need to provide a system for butt-glazing curtain wall panels which allows the panels to be installed in any sequence and which permits convenient reglazing of any panel.
There is also a need to provide a system for butt-glazing curtain wall panels which is easily adapted to a wide variety of mullion configurations.